An improved method and system for generating a frame check sequence. A multiple-bit data string, m, is received in which m is of the form:
m is thereafter parsed into multiple subframes of the form:
and
The subframes are padded with zeros resulting in subframes of the form:
and
A partial check sum is then generated for each of the multiple subframes. Finally, each of the partial check sums are added together such that a frame check sequence for m is obtained.
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1. A method for generating a frame check sequence for a data packet, wherein said data packet is composed of a string of binary bits, said method comprising the steps of:
parsing said data packet into m subframes each containing at least N bits and no more than N+1 bits that are interleaved at regular intervals with bits contained within the other m-1 subframes; computing a partial check sum for each of said m subframes; and adding each of said partial check sums such that a frame check sequence for said data packet is obtained.
6. A system for verifying the integrity of transmission of a data frame composed of a string of binary bits, said system comprising:
data processing means for parsing said data frame into m subframes each containing at least N bits and no more than N+1 bits that are interleaved at regular intervals with bits contained within the other m-1 subframes; means for computing a partial check sum for each of said m subframes; and a modulo-2 no carries adder for adding said partial check sums such that a frame check sequence for said data frame is obtained.
10. A program product stored in signal bearing media for generating a frame check sequence for a data packet, wherein said data packet is composed of a string of binary bits, said program product comprising:
instruction means for parsing said data packet into m subframes each containing at least N bits and no more than N+1 bits that are interleaved at regular intervals with bits contained within the other m-1 subframes; instruction means for computing a partial check sum for each of said m subframes; and instruction means for adding each of said partial check sums such that a frame check sequence for said data packet is obtained.
2. The method of
3. The method of
utilizing modulo-2 no-carries arithmetic to divide an ith one of said m subframes by a common divisor, P, wherein P is a k+1 bit binary number; and in response to said modulo-two division, obtaining a remainder, Ri, wherein Ri is utilized as a partial check sum associated with said ith subframe.
4. The method of
5. The method of
7. The system of
8. The system of
a modulo-2 no-carries divider for dividing an ith one of said m subframes by a common divisor, P, wherein P is a k+1 bit binary number.
9. The system of
11. The program product of
12. The program product of
instruction means for utilizing modulo-2 no-carries arithmetic to divide an ith one of said m subframes by a common divisor, P, wherein P is a k+1 bit binary number; and instruction means responsive to said modulo-two division, for obtaining a remainder, Ri, wherein Ri is utilized as a partial check sum associated with said ith subframe.
13. The program product of
14. The program product of
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1. Technical Field
The present invention relates in general to managing communications networks, and in particular, to a method and system for providing increased flexibility in processing data packets. More particularly, the present invention relates to a method and system for computing a frame check sequence (FCS) for a partitioned data packet. Still more particularly, the present invention relates to implementing Cyclic Redundancy Checks (CRCs) utilizing the inherent flexibility of modulo-2 arithmetic with no carries to provide a Cyclic Redundancy Check (CRC) that is adaptable with existing data processing structures and methods.
2. Description of the Related Art
CRC is a well known method for determining the presence of errors in digital transmissions in which discrete units of data, known as packets are delivered. The fundamental principle upon which CRC is based can be expressed equivalently in one of three ways. First, CRC can be described in terms of division of binary numbers. Second, as described by Boudreau et al. in U.S. Pat. No. 3,872,430, CRC may be performed utilizing a division of polynomials. Third, the utility in implementing CRCs is often realized by designing special check circuits in which Exclusive Or (XOR) and other elementary binomial operands generate frame check numbers for use during CRC.
Several types of packet-oriented data transmission systems are currently available. Token Ring, Ethernet, Asynchronous Transfer Mode (ATM), and Synchronous Optical Network (SONET) are examples of such systems which employ error detection and correction techniques such as CRC. When an information packet (sometimes referred to as a "frame" or "cell") is delivered from a source node to a destination node the receiver will utilize CRC to verify integrity of the transmission. To verify an accurate and successful transmission of an n-bit data packet, M, in accordance with conventional CRC methodologies requires two fundamental steps. First, a divisor P having n+1 bits is selected. For example, and with reference to Spragins p. 279, the divisor utilized in accordance with the IEEE 802 standard is the 33-bit number known as "CRC-32", as follows (the dot "." is for visual convenience only):
The next step is to append n 0-bits to the end of the data sequence M. This is equivalent to multiplying M (regarding M as a binary number) by 2n. Data sequence M is then divided by P utilizing modulo-2 arithmetic with, no carries and the remainder, R, is the Frame Check Sequence (FCS) of M. This FCS is then appended to the end (right) of M without the added 0-bits to produce the frame to be transmitted T. If T is correctly transmitted and then divided by P, the remainder is the n-bit number having all zero entries.
Various methods for employing CRC and computing a FCS are well known to those skilled in the art and for a further explanation of conventional CRC methods, reference is made herein to Boudreau et al. U.S. Pat. No. 3,872,430 Stallings, pp. 164-171, and Spragins, Hammond, Pawlikowski, p.279. These references provide a more detailed explanation of CRC calculations and are incorporated herein by reference.
Computation of a FCS for a lengthy data string is cumbersome and hardware intensive. It is therefore often desirable to divide the computation of a FCS for a data packet into several subcomputations which are faster and which impose a lesser degree of hardware overhead. Several techniques are known for performing such CRC computations on subdivided portions of the original data packet. U.S. Pat. No. 5,410,546 (Boyer et al.) and U.S. Pat. No. 5,325,372 (Ish-Shalom), describe one such approach in which partial CRC remainders (adjustment codes) are stored in a table. In this manner, the complete CRC check sequence (sometimes referred to as "check sum") may be reconstructed utilizing a software implementation that constructs a FCS from the partial CRC remainders. Such methods may result in lower computation time but do not necessarily reduce hardware and software overhead. In addition, these "table lookup" methods do not support multiple interleaved data streams and are therefore insufficient when utilized with asynchronous systems such as Asynchronous Transfer Mode (ATM).
It can therefore be appreciated that a need exists for an improved CRC computation methodology that capitalizes on existing logic structures to calculate and subsequently combine partial CRCs to form a packet CRC. Such a method and system, if implemented would reduce the overhead required for generating check sequences that provides the flexibility inherent in utilizing partial CRCs.
It is therefore an object of the invention to provide an improved method and system for managing data communications.
It is another object of the invention to provide a method and system for providing increased flexibility in processing data packets.
It is still another object of the invention provide a method and system for computing a frame check sequence (FCS) for a partitioned data packet.
It is a further object of the invention to provide a method and system for implementing Cyclic Redundancy Checks (CRCs) utilizing the inherent flexibility of modulo-2 arithmetic to provide a CRC that is adaptable with existing data processing structures and methods.
The above and other objects are achieved as is now described. An improved method and system for generating a frame check sequence are disclosed. In the preferred implementation, a multiple-bit data string, M, is received in which M is of the form:
M is thereafter parsed into multiple subframes of the form:
and
The subframes are padded with zeros resulting in subframes of the form:
00cn000cn-1000cn-20 . . . 00c2000c10;
and
A partial check sum is then generated for each of the multiple subframes. Finally, each of the partial check sums are added together such that a frame check sequence for M is obtained. In this manner, the sum of the partial check sums is guaranteed to be the same as the check sum for the original complete data packet.
The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
The computations required for obtaining or verifying a FCS require the use of substantial sequential and combinatorial logic. Data registers of at least 32 bits are typically required for performing the arithmetic operations entailed in computing a FCS. Telecommunications nodes such as switches and bridges are often designed or constructed so that incoming or outgoing data packets are divided into subparts before being processed within the node or released for transmission. The present invention introduces a new method and system for computing a CRC code, referred to interchangeably as a frame check sequence (FCS), by regularly partitioning the original data packet into subframes and obtaining partial FCSs therefrom. The partial FCSs are subsequently added to obtain the CRC for the original data packet. The steps required accomplish this are explained with reference to
With reference now to the figures, and in particular with reference to
As seen in
With reference now to
In order to verify integrity of data transmission of data packet 200, a FCS for data packet 200 must be generated and stored for future comparisons. After being initially generated, this number is transmitted as an attachment to data packet 200. The node receiving data packet 200 will then recompute the FCS in accordance with CRC convention and compare the newly computed value to the FCS received with data packet 200. If the newly computed version conforms with the FCS received, then correct transmission of data packet 200 has been verified. If there is a disparity, an error is indicated and the data packet may be retransmitted. Utilizing this approach, transmission errors in the information fields and header or flag fields of a data packet are detected.
As illustrated in
In an important feature of the present invention, the FCS obtained by adding remainders R1, R2, R3, and R4 will be equal to the FCS that would be obtained by dividing the entire original data packet 200 by P. A theorem in support of this result is provided and explained in greater detail in Appendix A. This feature of the present invention may be advantageously applied in the context of computing CRC codes for asynchronous data transfer technologies such as ATM in which multiple, regularly interleaved data streams must be accommodated.
Next, the data packet is partitioned, or parsed, into subframes as depicted at step 306. A preferred method of such parsing is illustrated with reference to data packet 200 and subframes 202, 204, 206, and 208 of FIG. 2 and also Appendix A. Thereafter, and as illustrated at step 308, the subframes are padded with zeros or some other suitable uniform value such that each subframe achieves the dimensions of the original data frame received at step 304. Modulo-2, no carries division is then performed on each subframe utilizing a common divisor such as CRC-32 as depicted at step 310. Each such division will result in a remainder, or "partial FCS" corresponding to each padded subframe. Next, as illustrated at step 312, the remainders are added together utilizing modulo-2, no carries arithmetic to obtain the FCS for the complete data packet.
Steps 313 through 320 depict a sub-process within methodology 300, in which the FCS may be verified upon receipt following delivery to a destination node. In accordance with conventional CRC methodologies, and as illustrated at step 313, the FCS is appended to the original data packet to form a transmission packet, T. Step 314 depicts reception of T at a destination node which subsequently performs a CRC check (steps 315 through 320) to verifying integrity of the transmission. In accordance with convention CRC methods, the verification performed as illustrated at step 315 is essentially a modulo-2, no carries division of T by the original common divisor, P. If an invalid result (non-zero) is detected at step 316, an error will be posted and an error signal indicating the need to retransmit the data packet will be posted by the receiving node.
It should be noted that the present invention provides a method of obtaining the FCS that is impervious to the sequential order in which the subframes are sent or received. The present invention is therefore adaptable to systems which utilize multiple, regularly interleaved data streams such as ATM. The capability to asynchronously complete the CRC calculation from partial results is a key feature of the present invention. The subframes must be collected and each divided by a common divisor P. The four remainders are then added together to construct the CRC for the complete data packet. The CRC is then appended to the data packet to form a complete frame, T, which is then transmitted to a receiver. Upon receipt of T, the receiver will verify correct transmission by dividing T by the common divisor and if the remainder is zero, transmission has been verified as error free. It should be noted that although computation methodology 300 as well as the supporting description in Appendix A describe computing a FCS for a data packet partitioned in four parts, the method and system of the present invention as described herein is applicable to a data packet partitioned by any integer greater than one.
The present invention utilizes the inherent flexibility of modulo-2 arithmetic (no carries addition and subtraction) to provide a method and system for computing a FCS for an entire data packet utilizing modulo-2 operations performed on subframes of the packet. Because the FCS derived from the subframes is the same as that obtained by conventional computations, the present invention may be seamlessly incorporated into existing CRC systems.
It is important to note that, while the present invention has been, and will continue to be, described in the context of a fully functional telecommunications subsystem, those skilled in the art will appreciate that the present invention is capable of being distributed as a program product in a variety of forms, and that the present invention applies equally regardless of the particular type of signal-bearing media utilized to actually carry out the distribution. Examples of signal-bearing media include: recordable-type media, such as floppy disks, hard disk drives, and CD ROMs, and transmission-type media such as digital and analog communication links.
Preferred implementations of the invention include implementations as a computer system programmed to execute the method or methods described herein, and as a program product. According to the computer system implementation, sets of instructions for executing the method and methods are resident in RAM of one or more computer systems configured generally as described above. Until required by the computer system, the set of instructions may be stored as a computer-program product in another computer memory, for example, in a disk drive (which may include a removable memory such as an optical disk or floppy disk for eventual utilization in disk drive).
The computer-program product can also be stored at another computer and transmitted when desired to the user's workstation by a network or by an external communications network. One skilled in the art can appreciate that the physical storage of the sets of instructions physically changes the medium upon which it is stored so that the medium carries computer-readable information. The change may be electrical, magnetic, chemical, or some other physical change. While it is convenient to describe the invention. in terms of instructions, symbols, characters, or the like, the reader should remember that all of these and similar terms should be associated with the appropriate physical elements. Thus, a method for implementing the present invention as depicted in
With reference now to
Subframes 406 are then padded with zeros as illustrated at step 203 of FIG. 2 and Appendix A and delivered to a divider depicted at block 408. As shown in bock 408, the divider performs modulo-2, no carries division on each of the padded subframes. For each such division operation, the dividend is the padded subframe, and, in accordance with conventional CRC techniques (See Stallings pp. 164-171), the divisor is a predetermined common divisor, comprised of k+1 bits. The remainder from the modulo-2, no carries division of subframe i by P will be utilized as the "partial check sum" for subframe i. After all N of such partial check sums 410 have been computed, they are added together within a modulo-2, no carries adder, or its functional equivalent illustrated at block 412. The sum of all N partial check sequences is the final frame check sequence 414 that is then associated with and appended to data packet 402.
While the invention has been particularly shown as described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims.
APPENDIX A
This appendix sets forth a general method of deriving a Frame Check Sequence (FCS) in accordance with the method and system of the present invention.
Modulo-2 addition with no carries is distributive, that is, x*(y+z)=x*y+x*z. Let a, b, c, and d be any four binary numbers, and let P be any nonzero divisor. Dividing P into a yields a quotient Q(a) and a remainder R(a). Thus a=Q(a)*P+R(a). Similarly, dividing P into b, c, and d, and designating F to be the sum a+b+c+d:
Thus,
Substituting padded subframes for a, b, c, and d in this example, a set of four remainders will be obtained. The four remainders that result from each division are then added together to obtain the FCS for data packet 200. That is, suppose a full frame:
is partitioned as:
an000an-1000an-2000 . . . a2000a1000 + | |
0bn000bn-1000bn-200 . . . 0b2000b100 + | |
00cn000cn-1000cn-20 . . . 00c2000c10 + | |
000dn000dn-1000dn-2 . . . 000d2000d1, | |
which, for illustrative purposes may be labelled as subframes Sa, Sb, Sc, and Sd respectively. In accordance. with the relation depicted in equation (2):
Thus the FCS for data packet 200 may be obtain in this manner from subframes 202, 204, 206, and 208. The process of CRC computation is thus modified by partitioning the data packet in a manner consistent with subframes 202, 204, 206, and 208.
Calvignac, Jean Louis, Verplanken, Fabrice Jean, Jeffries, Clark Debs
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